Many diseases, impairments, and injuries are presently treated by medical devices that must be able to pass through the skin in order to treat a patient. A variety of medical devices exist that must pass through the skin to allow transfer of fluids, gases, skeletal loading, power, or other ingests between the external environment and the internal environment of the body. In addition, many of these through-skin or “percutaneous” devices must also remain in place for long time periods, or indefinitely.
For instance, indwelling catheters are used to provide access to various body compartments. Central venous catheters (also known as central lines) are used in intensive care medicine to provide ingests such as drugs, fluids, nutrition, or blood products over a long time periods—usually several weeks or longer. The percutaneous catheter, comprising a long, flexible tube, is inserted into a large vein through an incision in the abdominal wall, chest wall, or upper extremity and threaded along the vein until the distal end reaches the heart. Central lines are also used for out-patient procedures such as long-term drug therapy and for hemodialysis, in which case the intention is that the catheter will remain in place for long time periods or permanently. An indwelling peritoneal catheter is used for drainage of abscises in advanced malignancy or liver disease, or to drain an abdominal abscess.
There are many diseases and medical conditions that require such percutaneous devices (such devices hereinafter also known as implants or percutaneous implants). For instance, patients with renal failure commonly require hemodialysis, which requires regular high-flow blood transfer that is presently provided through the surgical creation of an arteriovenus (AV) fistula. Unfortunately there are often problems maintaining continual access in these patients, and AV fistulas take time to mature before they can be used and often fail. In the interim, a chronic percutaneous catheter can be used to provide both arterial and venous access. However, long-term use of these percutaneous devices frequently results in infection. A device that permits long-term, infection-free access for the provision of hemodialysis would be extraordinarily useful to many patients who suffer renal failure.
Percutaneous mechanical cardiac assist devices include the intra-aortic balloon pump (IABP) and percutaneous ventricular assist devices (pVADs). The IABP comprises a catheter with an inflatable balloon at the distal end. The catheter is inserted into the femoral artery until the balloon is positioned in the descending thoracic aorta. The balloon can be inflated (in order to increase cardiac and systemic blood flow) and deflated (to decrease cardiac output, ventricular wall tension, and myocardial oxygen demand) in synchrony with the cardiac cycle, as required. pVADs are pumps that are surgically inserted into the ventricles of the heart to improve heart function in heart failure, to treat cardiogenic shock (for example, after cardiac arrest) or to maintain heart function while the patient is waiting for a heart transplant. pVADs may be inserted into the left ventricle (LVAD devices), right ventricle (RVAD) or into both ventricles (BiVAD). A percutaneous drive line is necessary to provide electrical power to operate the pump of either an IABP or a pVAD. Such devices may remain in place for weeks or months until either the heart regains full function or a transplant is performed or, increasingly, are intended to remain in place for the life of the patient.
Yet another example of a percutaneous device is a nephrostomy tube, a percutaneous catheter that is passed through the body wall and renal parenchyma and terminates in the renal pelvis, where urine collects prior to entering the ureter. The catheter is used to allow urinary drainage when the ureter or bladder is obstructed by injury or malignancy, or is not under voluntary control, for example, after a spinal cord injury. The nephrostomy tube can also be used to deliver drugs, such as chemotherapeutic agents, to the renal pelvis. Future medical devices such as a wearable artificial kidney, lung, or liver will require placement of permanent percutaneous lines to allow exchange of blood and/or gases between the body and the artificial organ.
The use of percutaneous implants extends into the prosthetic field. The direct attachment of a prosthetic limb to the residual bone of a patient's amputated limb provides many advantages over traditional socket attachment, including increased comfort of and utility to the patient. In a direct prosthetic attachment, a distal portion of residual bone of the patient's remaining limb (known as the “abutment”) protrudes through the skin to allow attachment of the prosthetic device. A metal rod is inserted into the abutment. Such direct skeletal attachment provides a rigid connection of the prosthesis to the body and eliminates the need for a socket—a device which is worn over the residual limb or torso to permit attachment and suspension of the prosthesis, and use of which can cause pain, discomfort, or infection to an amputee. Direct attachment of the prosthesis to the residual bone also allows for better control of the prosthesis in space and more natural loading of the skeletal system. It provides an increased range of motion and improved mobility, allows increased activity levels, and, in the case of lower limb amputation, improves gait. An additional advantage is the provision of osseoperception, a sensation arising from mechanical stimulation by the bone-anchored prosthesis, which is transduced by mechanoreceptors in the muscle, joint, or surrounding tissues (Klineberg et al. 2005) such that the user perceives sensations of vibration from the prosthetic device.
A major impediment to the use of percutaneous medical devices like the ones described above is that a chronic open wound is created where the device passes through the skin. Infection of the exit wound created by the device is a frequent occurrence. Infection is an important consideration in any long- or short-term percutaneous implant. Infection may include superficial skin infections, deep tissue infection, or infection of bone (osteomyelitis). Bloodstream infections are a major cause of hospitalization and death in hemodialysis patients and significantly increase the cost of treatment. Cardiac assistive devices have very high rates of infection that endanger the patients who use them. Despite the potential benefits for prosthesis users, high infection rates have impaired the FDA approval of methods for direct skeletal attachment of prostheses. In sum, an infection in a wound surrounding a percutaneous device reduces quality of life and threatens a patient's health, and its treatment increases health care costs and may necessitate removal of the percutaneous device.
The inability to create a stable, permanent seal between the percutaneous implant and the surrounding skin causes a chronic exit wound that allows infection to develop. Two major factors contribute to the disruption of the skin-implant interface. The first factor involves different stiffness measurements between the implant and the surrounding skin. Human soft tissue, such as skin, has a low stiffness and a corresponding high compliance. When intact skin is pulled or stretched, its natural low stiffness/high compliance characteristics disperse the resulting forces over a large area of skin and subcutaneous tissue. Percutaneous implants, on the other hand, are made from material such as plastic or titanium that is more stiff/less compliant than human soft tissue. A rigid implant (such as one made from hard plastic or titanium) or a semi-rigid implant (such as one made from softer plastic material, such as that used in plastic tubing) allows relative motion between the implant and the surrounding soft tissue. When a rigid or semi-rigid percutaneous implant is in place, any pulling or stretching force applied to the implant is concentrated at the skin edge that surrounds the implant. The mechanical forces that pull on the skin lead to differential motion between the skin and the implant. This results in regression of the edge of the skin surrounding the implant, leading to a larger wound around the implant. Relative motion between the implant and surrounding soft tissues can also lead to fluid accumulation, which predisposes the skin-implant interface to infection. The second factor contributing to the disruption of the skin-implant interface is the apparent down growth of epithelial tissue into the wound, also known as the “marsupialization” of skin. Marsupialization can cause a pocket, or sulcus, to form around the implant, which allows debris and moisture to accumulate, facilitating infection and destabilizing the implant.
Previous attempts to prevent infection in the open wound surrounding a percutaneous device have included various strategies. Biological strategies include protein-coated devices, drug-releasing devices, and use of antimicrobial or antiseptic agents and dressings. Engineering strategies include new implant materials, new implant surface topographies, new implant structures or shapes, and addition of a stabilizing flange or cuff at or below the skin surface. For example, titanium implants with porous surfaces, or implants with hydroxyapatite coated subcutaneous flanges are known. Other subcutaneous systems, including implant umbrellas, metal flange implants, and subdermal meshes, provide greater surface area, which that increases the chances of infection of the implant. The goal of these subcutaneous implant approaches is to allow subdermal tissues to scar down on or into the implant, thus buffering the implant site against skin-stretching forces. For percutaneous prosthesis attachment, surgical strategies include attempts to attach skin directly to the bone. Some previous approaches have resulted in decreased infection rates in animal models, but all are still plagued by prohibitively high rates of skin marsupialization and infection. High infection rates have been seen in human trials.
Prior attempts to resolve compliance differences between skin and implant do not address the limitations overcome by the present invention. When a stiff material, such as a metal rod, sticks through the skin, a key issue is that forces are concentrated at that stiff material. The skin is pliable and would naturally stretch when pulled. However, the stiff rod prevents this stretching and the shear forces are all applied to the opposite side of the rod, just a few millimeters off the outer circumference. This causes high forces on the skin on that side of the rod. The skin erodes due to the pressures caused by these high shear forces and the wound gets larger as shown in FIG. 8. If the shear forces are always applied from one direction, the wound tends to elongate. If the shear forces are applied in many directions then the entire wound margin suffers and the wound increases in diameter globally. Furthermore, because they are not modular or because they are implanted directly into the skin, the prior art systems are difficult to remove and replace if a serious wound infection occurs or if the implant itself becomes infected.
Negative pressure has become a regularly applied therapy for many types of wounds. Negative pressure is sub-atmospheric pressure, and negative pressure therapy involves the application of sub-atmospheric pressure to a wound environment. Negative pressure therapy can facilitate wound healing through several primary mechanisms, including: (1) macrodeformation—drawing the wound edges together; (2) stabilization of the wound environment; (3) edema reduction and removal of wound exudates; and (4) microdeformation, which is stretch applied at the cellular level that stimulates cell growth. Secondary benefits attributed to negative pressure therapy include increased angiogenesis, enhanced formation of granulation tissue, and a reduction in the microbial bio-burden of the wound. Chronic application of negative pressure to a wound caused by a percutaneous device may prevent or limit the degree of marsupialization that occurs at the skin-implant interface. The prior art additionally fails to adequately address infection that is likely to develop in or around a percutaneous wound. Wound irrigation cannot be performed with conventional wound vacuum systems. The wound vacuum must be removed, the wound manually irrigated, and then the wound vacuum replaced.